To improve energy conversion efficiency of power modules, fabrication of devices with use of silicon carbide (SiC) base chip or gallium nitride (GaN) base chip in place of a silicon base chip is regarded as a next-generation technique for power semiconductor switches. For example, SiC devices provide low conductor resistance, fast switching speed, and small power loss, and can operate at a high temperature; thus, the SiC devices comply with the development trend of high-power devices. However, when silicon-based power modules are in operation, the junction temperature Tj is around 150° C., which is still within the acceptable temperature range of a lead-free solder material (e.g., Sn3.0Ag0.5Cu); when the SiC device modules are introduced, the junction temperature Tj increases to above 175° C. even for medium-to-low wattage power control modules, and the initial melting temperature of Sn3.0Ag0.5Cn is only 217° C. Therefore, on such temperature conditions, serious creep effects may take place, which is disadvantageous for maintaining mechanical strength of junction points and satisfying requirements for long-term reliability, not to mention products such as car power modules which are applied at the junction temperature Tj of 250° C. Accordingly, in a SiC power module package, the use of high temperature lead-free solder is one of the factors in deciding quality and lifespan of the products.
In view of the above, high temperature lead-free solders or high temperature resistant die bonding processes have been developed by many research organizations. Currently, high temperature lead-free solders, such as AuSn-based solders, BiAg-based solders, Ag paste sintering, and Zn-based solders, have been widely used.
Compared to other high temperature solders, the Zn-based solder characterized by low costs, proper melting range, and good thermal and electrical conductivity is rather suitable for being applied to power module packaging in replacement of other high temperature solders. However, the Zn-based solder is prone to oxidation. Besides, in a high temperature bonding process, the Zn-based solder is readily reacted with other metal (e.g., copper) to form intermetallic compounds, such as CuZn5 and Cu5Zn8, thereby impairing long-term reliability of the power modules.